HYALONEMA (OONEMA) CRASSIPINULUM. 335 



The dermal superficial pinules have distal rays 250-790 n long. Their 

 length frequency-curve exhibits, Uke that of the gastrals, two very distinct 

 ele^•ations, so that also among these pinules two kinds, a large and a small one, 

 must be distinguished. 



The large dermal pimiles (Plate 92, fig. 6) are pentactine. Their distal ray 

 is usually straight and 500-790 ^ long, most frequently GOO-650 n. It is 15-24 ^ 

 tliick at the base, and thickened above. At its point of maximum thickness, 

 which lies a little above the middle, it attains a transverse diameter about twice 

 as great as its basal thickness. The distal ray ends with a low and broad termi- 

 nal cone. Its spinulation is similar to that of the gastral pinules. The spines 

 are proportionately smaller. The maximum diameter of the distal ray, together 

 with the spines, is 50-90 m- The lateral rays are cylindroconical, and 45-95 m 

 long. They bear small, sparse, broad, and low, conical spines. 



The small dermal pinules (Plate 92, figs. 7, 18) are rather similar to the 

 larger ones and, like them, all pentactine. Their dimensions are: — distal 

 ray, length 250-440 /j, basal thickness 10-17 n, maximum thickness together 

 with the spines 28-65 m; lateral rays, length 45-70 m- 



The canalaY pinules (Plate 92, figs. 16, 17) are pentactine or hexactine. 

 The distal ray is straight, 120-150 fx long, and 5-9 fi thick at the base. It is 

 slightly thickened above, gradually attenuated to a fine point, and bears rather 

 sparse, small, straight spines directed obliquely upwards. Its maximum thick- 

 ness, together with the spines, is 7-30 m- The lateral rays are 45-95 n long; the 

 proximal, when present, is 50-70 fi. Both the lateral rays and the proximal 

 are spiny. 



There seems to be no great difference between the hypodermal and hijpo- 

 gaslral pentactines. Both have straight, conical, blunt rays. The lateral 

 rays are 230-550 /i long; the proximal ray is 400-700 ju long and 15-60 ^ thick 

 at the base. 



The hexactine megascleres generally have fairly equal rays. In some, two 

 opposite rays are a little longer than the others, but the difference ne^'er appears 

 to be great. The hexactine megascleres observed are 0.8-1.2 mm. in diameter. 

 The basal thickness of their rays is 25-30 ^i. 



The amphioxes of the dermal and gastral membranes (pore-sieve reticula- 

 tions) and the choanosome are centrotyle, straight or curved, sometimes very 

 considerably, and 0.7-1.7 mm. long. Near the middle they are 8-29 ^ thick. 

 The central tyle is 10-34 /x in diameter, that is 2-5 // more than the adjacent 

 parts of the spicule. 



